1. Field of the Invention
The present invention relates to a control system for controlling a parallel hybrid vehicle.
2. Description of the Related Art
Some parallel hybrid vehicles have an internal combustion engine as a primary propulsive power unit, an electric generator motor operable alternatively as an electric motor and an electric generator, and an electric energy storage device such as a battery or a capacitor for supplying stored electric energy to and receiving electric energy for storage from the electric generator motor. When the hybrid vehicle is accelerating, the generator motor is supplied with stored electric energy from the electric energy storage device and operated as an electric motor to generate an assistive output power which is added to the output power (vehicle propulsion power) generated by the engine. When the hybrid vehicle is decelerating, the generator motor is operated as an electric generator by the kinetic energy of the hybrid vehicle to regenerate electric energy, which is stored in the electric energy storage device.
One known control system for a hybrid vehicle of the type described above is disclosed in Japanese laid-open patent publication No. 8-182113, for example. According to the disclosed control system, an integrated charged quantity which represents an integrated quantity of electric energy regenerated by the motor generator and charged into the electric energy storage device is regarded as an allowable discharged quantity that represents a quantity of electric energy which can be discharged from the electric energy storage device to the motor generator when the motor vehicle is to be operated as the electric motor. When the motor generator is operated as the electric motor, the motor generator is controlled so that an integrated discharged quantity which represents an integrated quantity of electric energy discharged from the electric energy storage device will be kept within the allowable discharged quantity, i.e., the supply of electric energy to the motor generator is stopped when the integrated discharged quantity reaches the allowable discharged quantity.
In the above conventional control system, therefore, only the electric energy which has been stored in the electric energy storage device by regenerative operation of the motor generator is used to operate the motor generator as the electric motor when the hybrid vehicle is accelerating, for thereby achieving a balance between charged and discharged quantities of electric energy.
However, since only the electric energy which has been stored in the electric energy storage device by the regenerative operation of the motor generator is used to operate the motor generator as the electric motor for generating an assistive output power to be added to the output power generated by the engine, if the hybrid vehicle is in a situation where the generator motor does not operate frequently in a regenerative mode, such as when the hybrid vehicle runs uphill, then the motor generator cannot operate as the electric motor for generating an assistive output power to be added to the output power generated by the engine. In such a situation, it is difficult to increase the fuel economy of the engine effectively.
Furthermore, if the electric energy storage device is nearly fully charged when the hybrid vehicle starts running, then the generator motor cannot actually operate in the regenerative mode because the electric energy storage device needs to avoid being excessively charged. In such a situation, consequently, the motor generator also fails to operate as the electric motor for generating an assistive output power to be added to the output power generated by the engine. In this situation, furthermore, the electric energy maintained by the electric energy storage device cannot effectively be utilized, and the kinetic energy of the hybrid vehicle upon deceleration thereof cannot sufficiently be recovered by the regenerative operation of the generator motor. As a result, it is difficult to sufficiently increase the efficiency with which the hybrid vehicle utilizes the available energy.